The present invention relates to the purification of acetic acid. More particularly, it relates to the removal of water from acetic acid streams produced using a catalytic system containing an iodine component.
A process has recently been developed for the production of carboxylic acids such as acetic acid by the reaction of an alcohol or an ester, or ether and halide derivatives thereof, with carbon monoxide in contact with catalyst systems formed on mixing of a rhodium or iridium component and a halogen component in the presence of carbon monoxide (U.S. pat. Nos. 3,769,329 and 3,772,380). In actual practice, the halogen component is generally iodine in the form of an alkyl iodide, notably methyl iodide, or hydrogen iodide. The acid product thus produced contains water and residual amounts of the iodine component as contaminants. In order that the acid produced in such a process can be employed in further reactions and other uses, it must be freed from any water present as well as from the small amounts of iodine present. The usual methods of purification to recover the acetic acid and removal of the halogen contaminants involves a series of distillations. Water can be removed, for example, by the distillation techniques described in U.S. Pat. No. 3,769,177 and 3,791,935. However, it has been found that water cannot be removed by these methods without simultaneous removal of the alkyl iodide such as methyl iodide because methyl iodide does not remain in the liquid phase, either in acetic acid or water, and is taken overhead with the water. Since economics dictate that the methyl iodide must be reused, the water-methyl iodide stream is generally recycled to the reactor. Also, water removal at the levels indicated requires acid removal as well, the acid/water weight ratio being about 1:1. It is obvious, of course, that this stream also has to be recycled to recover the acid values. This operation is satisfactory until water buildup in the system due to lack of reaction as well as leaks result in excess amounts of water in the feed to the drying column which create a bottleneck in the drying operation forcing a cutback in rates which considerably slows down production of pure acid. One obvious corrective action to reduce the water content of the feed would be to discard water from the overhead stream. Discarding water also means discarding methyl iodide, however, and creates a disposal problem in addition to the adverse economic effect resulting when methyl iodide is not recycled. It is; accordingly, an object of the present invention to provide a method for the removal of excess water during distillation of a crude acetic acid product to remove water and methyl iodide therefrom which does not result in any loss of methyl iodide or create any disposal problems and wherein the amount of acetic acid simultaneously removed is such that the resulting acid-water stream can be disposed of or rectified at low cost as desired.